Field of the Invention
Embodiments of the present invention relate generally to a pattern inspection apparatus, and a pattern inspection method. More specifically, for example, embodiments of the present invention relate to a pattern inspection technique for inspecting pattern defects of an object serving as a target workpiece or “sample” used in manufacturing semiconductor devices, and to an inspection apparatus for inspecting defects of minute patterns formed on a photomask, wafer, or liquid crystal substrate used in manufacturing semiconductor elements or liquid crystal displays (LCDs).
Description of Related Art
In recent years, with the advance of high integration and large capacity of large-scale integration (LSI) circuits, the line width (critical dimension) required for circuits of semiconductor elements is becoming progressively narrower. Such semiconductor elements are manufactured by circuit formation of exposing and transferring a pattern onto a wafer by means of a reduced projection exposure apparatus known as a stepper while using an original or “master” pattern (also called a mask or a reticle, hereinafter generically referred to as a mask) with a circuit pattern formed thereon. Then, in fabricating a mask for transfer printing such a fine circuit pattern onto a wafer, a pattern writing apparatus capable of writing or “drawing” fine circuit patterns by using electron beams needs to be employed. Pattern circuits may be written directly on the wafer by the pattern writing apparatus. Also, a laser beam writing apparatus that uses laser beams in place of electron beams for writing a pattern is under development.
Since LSI manufacturing requires a tremendous amount of manufacturing cost, it is crucial to improve its yield. However, as typified by a 1-gigabit DRAM (Dynamic Random Access Memory), the scale of patterns configuring an LSI is in transition from on the order of submicrons to nanometers. One of major factors that decrease the yield of the LSI manufacturing is due to pattern defects on the mask used for exposing and transfer printing an ultrafine pattern onto a semiconductor wafer by the photolithography technology. In recent years, with miniaturization of dimensions of LSI patterns formed on a semiconductor wafer, dimension to be detected as a pattern defect has become extremely small. Therefore, a pattern inspection apparatus for inspecting defects on a transfer mask used in manufacturing LSI needs to be more highly accurate.
As an inspection method, there is known a method of comparing an optical image obtained by imaging a pattern formed on a target object or “sample” such as a lithography mask at a predetermined magnification by using a magnification optical system with design data or an optical image obtained by imaging the same pattern on the target object. For example, the methods described below are known as pattern inspection methods: the “die-to-die inspection” method that compares data of optical images of identical patterns at different positions on the same mask; and the “die-to-database inspection” method that inputs, into an inspection apparatus, writing data (design pattern data) generated by converting pattern-designed CAD data to a writing apparatus specific format to be input to the writing apparatus when a pattern is written on the mask, generates a design image (reference image) based on the input writing data, and compares the generated design image with an optical image (serving as measurement data) obtained by imaging the pattern. In such inspection methods for use in the inspection apparatus, a target object is placed on the stage so that a light flux may scan the target object as the stage moves in order to perform an inspection. Specifically, the target object is irradiated with a light flux from the light source through the illumination optical system. Light transmitted through the target object or reflected therefrom forms an image on a sensor through the optical system. The image captured by the sensor is transmitted as measurement data to the comparison circuit. After performing position adjustment of images, the comparison circuit compares measurement data with reference data in accordance with an appropriate algorithm, and determines that there exists a pattern defect if the compared data are not identical.
In the inspection apparatus, the target object on the stage is irradiated with an inspection light, and its transmitted light or reflected light is input into the optical system. As a method for detecting the position of the stage, a laser length measuring method using a laser interferometer is known, for example. Then, x and y positions of the target object and a rotation angle of the target object are calculated using a result of the laser length measuring. However, laser beams are affected by fluctuation of air. Therefore, it is necessary to average the result of the laser length measuring, with spending a predetermined time. Thus, it is difficult for the laser length measuring method to rapidly and highly accurately perform position measurement.
As a measurement method less susceptible to influence of air flow and/or temperature change, a two-dimensional scale is considered and examined. Although not concerning the inspection apparatus, there is disclosed a method for measuring the position of a wafer stage by placing a wafer in the center of the wafer stage of an exposure apparatus, attaching (fixing) a head to each of the four corners of the wafer stage, and upwardly emitting a measurement beam from the head to a scale plate arranged around the irradiation opening of an exposure light (for example, refer to Japanese Unexamined Patent Application Publication (JP-A) No. 2014-017501). However, with respect to the inspection apparatus, since the region where the target object is placed serves as an optical path, it is difficult to arrange the target object in the center of the stage whose gravity is located on the working point of the stage thrust. The stage posture tends to tilt by pitching and/or rolling of the stage at the time of driving. If the head for emitting measurement beams is placed on the stage, beam irradiation positions may be shifted by the tilt of the stage. Therefore, an error may occur at the measurement position. Although it is desired to perform position measurement with great accuracy in order to conduct a highly precise inspection, a sufficient solution has not been established yet.